The present invention relates to an electromagnetic clutch which transmits power by utilizing a magnetic force generated by the magnetic flux of a permanent magnet and cuts off power transmission by energizing an excitation coil.
Various types of electromagnetic clutches which transmit power by a magnetic force generated by the magnetic flux of a permanent magnet (to be merely referred to as a magnetic force as well hereinafter) are conventionally proposed (see U.S. Pat. No. 3,263,784, Japanese Utility Model Publication No. 63-011394, and Japanese Patent Laid-Open No. 2007-333109).
A non-excitation actuation type electromagnetic clutch described in U.S. Pat. No. 3,263,784 includes a rotor attached to a rotating shaft, a hub rotatably attached to the rotating shaft through a bearing, an armature attached to the hub, and a permanent magnet and excitation coil which line up on the rotor in the axial direction. The rotor is formed of two members, i.e., an outer magnetic pole member and inner magnetic pole member which are integrally coupled by a compound made of a nonmagnetic material. The permanent magnet is interposed in a space formed between the flanges of the two magnetic pole members.
The permanent magnet forms a magnetic circuit in which a magnetic flux flows from the outer magnetic pole (the magnetic pole surface of the outer magnetic pole member) of the rotor to the inner magnetic pole (the magnetic pole surface of the inner magnetic pole member) of the rotor via the armature. The excitation coil forms a magnetic circuit in which a magnetic flux flows through a field core—an inner magnetic path member—the permanent magnet—an outer magnetic path member—the field core.
In the non-excitation actuation type electromagnetic clutch having the above structure, in a non-excitation state in which energization to the excitation coil is cut off, the frictional surface (magnetic pole surface) of the rotor magnetically attracts the frictional surface (magnetic pole surface) of the armature by the magnetic force of the permanent magnet, so that the rotor and armature are frictionally coupled to each other. For example, in a non-excitation actuation type electromagnetic clutch for a water pump, power of an engine is transmitted to a hub through a belt. Thus, while the engine is driven, the water pump is also driven. When cutting off power transmission to the water pump while the engine is driven, the excitation coil is energized, and a magnetic force generated by the magnetic flux of the excitation coil cancels the magnetic force of the permanent magnet. The armature is thus released from the magnetic force generated by the permanent magnet, so it separates from the rotor by a spring elastic restoration force, to cut off power transmission from the engine to the water pump.
In the non-excitation actuation type electromagnetic clutch described in Japanese Utility Model Publication No. 63-011394, a rotor is rotatably disposed on a stationary housing through a bearing. The rotor includes an inner cylindrical magnetic path portion (inner magnetic flux path portion), an outer cylindrical magnetic path portion (outer magnetic flux path portion), and a disc-like magnetic path portion (flange) which connects one end of the inner cylindrical magnetic path portion to one end of the outer cylindrical magnetic path portion. A pair of annular magnetic plates, a permanent magnet, and part of an excitation coil are incorporated in an annular groove defined by the three magnetic path portions.
The permanent magnet opposes the outer cylindrical magnetic path portion and inner cylindrical magnetic path portion at an appropriate gap and is sandwiched by the pair of annular magnetic plates. Of the pair of annular magnetic plates, one which is located on the deeper-end side than the permanent magnet in the annular groove opposes the inner surface of the disc-like magnetic path portion and that of the outer cylindrical magnetic path portion at an appropriate gap and is fixed to the outer surface of the inner cylindrical magnetic path portion. The other annular magnetic plate opposes the inner cylindrical magnetic path portion and excitation coil at an appropriate gap and is fixed to the inner surface of the outer cylindrical magnetic path portion. In the same manner as in the invention described in U.S. Pat. No. 3,263,784, when the permanent magnet magnetically attracts the armature to transmit power, the excitation coil is held in the non-excitation state. When cutting off power transmission, the excitation coil is energized and cancels the magnetic force of the permanent magnet in the same manner as in U.S. Pat. No. 3,263,784, thus cutting off power transmission.
In a power transmission mechanism described in Japanese Patent Laid-Open No. 2007-333109, in the same manner as in the electromagnetic clutch described in Japanese Utility Model Publication No. 63-011394, an excitation coil and a permanent magnet which is sandwiched by a pair of magnetic plates line up in the annular groove of a rotor in the axial direction. When transmitting power, the frictional surface of the rotor magnetically attracts an armature by the magnetic force of the permanent magnet. When cutting off power transmission, the excitation coil is energized similarly, so that the magnetic force of the permanent magnet is canceled and power transmission is cut off.
Each of the conventional electromagnetic clutches described in U.S. Pat. No. 3,263,784, Japanese Utility Model Publication No. 63-011394, and Japanese Patent Laid-Open No. 2007-333109 is usually employed as a power transmission device for a driven device such as a vehicle air-conditioning compressor or water pump. If, however, the rotating shaft of the driven device is locked (stopped) by an overload, or oil is externally attached between the frictional surfaces of the rotor and armature, abnormal slippage occurs between the rotor and armature, and the rotor rotates by slipping. When such slip rotation occurs, the frictional heat heats the rotor and armature instantly. Then, a belt that transmits the rotation of the driven device to the rotor may be burned, or the grease of the bearing that axially supports the rotor may flow out to seize the bearing, quickly rendering the electromagnetic clutch inoperative.
As a countermeasure for the slip rotation of the rotor caused by the overload or the like of the driven device, electromagnetic clutches described in Japanese Patent Laid-Open No. 57-051025, Japanese Utility Model Publication No. 59-027550, Japanese Patent Laid-Open No. 2006-200570, Japanese Patent Laid-Open No. 8-135686, and Japanese Utility Model Publication No. 59-005232 are known.
The electromagnetic clutch described in Japanese Patent Laid-Open No. 57-051025 which transmits power by the magnetic force of the magnetic flux generated by an excitation coil and includes a temperature fuse. When the frictional heat generated by slip rotation heats the temperature fuse and its vicinity, the fuse fuses by the heat and cuts off energization to the excitation coil, so an armature is released from a rotor.
In the electromagnetic clutch described in Japanese Utility Model Publication No. 59-027550, a housing for an input pulley and driven device is provided with a temperature detector and rotation pickup device opposing each other. When the temperature of the input pulley increases abnormally, the heat-shrinkable member of the temperature detector shrinks to cut off an output from the rotation pickup device. This cuts off energization to an excitation coil and causes the input pulley to idle.
The electromagnetic clutch described in Japanese Patent Laid-Open No. 2006-200570 includes a torque limiter which is connected to a clutch plate (armature) through an elastic member and rotates together with a rotating shaft. In the torque limiter, a hub flange is provided with a ball engaging groove, and a plate stacked on the flange is provided with an annular escape portion. A spherical member is interposed in the ball engaging groove. When the rotating shaft is locked by an overload, a large torque acts on the clutch plate, so the spherical member moves from the ball engaging groove to the escape portion, thereby releasing the clutch plate from the torque limiter. As a result, the clutch plate is separated from the rotor by the elastic force of the elastic member, and cuts off power transmission.
In the electromagnetic clutch described in Japanese Patent Laid-Open No. 8-135686, a damper mechanism connects an armature and hub to be separable. When the rotating shaft of a driven device is locked by an overload, a large torque acts on the elastic ring body (damper rubber) of the damper mechanism. Then, the elastic ring body elastically deforms and comes out from the gap between a pair of holding members, thereby disconnecting pulley and hub from each other.
In the electromagnetic clutch described in Japanese Patent Laid-Open No. 57-051025, however, power is transmitted and cut off by energizing and deenergizing an excitation coil. If the electromagnetic clutch is a so-called self-hold electromagnetic clutch which transmits power by connecting a rotor and armature by the electromagnetic force of a permanent magnet, a temperature fuse as shown in Japanese Patent Laid-Open No. 57-051025 cannot be employed.
Even the self-hold electromagnetic clutch can cut off power transmission to the rotating shaft of the driven device if it additionally includes a rotation pickup device or torque limiter mechanism as described in Japanese Utility Model Publication No. 59-027550 or Japanese Patent Laid-Open No. 2006-200570. In this case, however, the rotation pickup device must be mounted in the stationary housing of the driven device such that it opposes the pulley. Accordingly, an installation space is required, and the assembly operation becomes cumbersome. Also, the number of constituent components of the electromagnetic clutch increases, leading to a high product cost.
As the electromagnetic clutch described in Japanese Patent Laid-Open No. 8-135686 requires the damper mechanism, its installation space is larger than that of the spring type electromagnetic clutch described in Japanese Patent Laid-Open No. 2007-333109, Japanese Utility Model Publication No. 59-027550, Japanese Utility Model Publication No. 59-005232, and the like which transmits torque by utilizing rolling force generated by torsional deformation of a clutch spring. Also, the number of components increases, so the product cost increases.